An internal combustion engine having a electronically controlled fuel injection apparatus with an air-fuel ratio feedback correction control function employs an air-fuel ratio learning and controlling apparatus such as disclosed in Japanese Unexamined Patent Publication Nos. 60-90944 and 61-190142.
The air-fuel ratio feedback correction control function calculates a basic fuel injection quantity Tp according to engine operating parameters such as an inlet airflow quantity Q and an engine rotation speed N which influence the quantity of intake air to the engine. At the same time, an oxygen sensor disposed in an engine exhaust system determines if an actual air-fuel ratio is rich or lean with respect to a target air-fuel ratio (a theoretical air-fuel ratio), and according to the whether it is rich or lean, an air-fuel ratio feedback correction coefficient LMD is set. The basic fuel injection quantity Tp is corrected according to the air-fuel ratio feedback correction coefficient LMD, thereby carrying out a feedback control and adjusting of the quantity of fuel to be supplied to the engine, to bring the actual air-fuel ratio close to the target air-fuel ratio.
A deviation of the air-fuel ratio feedback correction coefficient LMD from a reference value (a target of convergence) is learned for each of divided regions of an engine operating range, to determine a learned correction coefficient KBLRC by which the basic fuel injection quantity Tp is corrected to match the basic air-fuel ratio substantially with the target air-fuel ratio before applying the correction coefficient LMD. Thereafter, a further feedback correction of the air-fuel ratio with the correction coefficient LMD is carried out to provide a final fuel injection quantity Ti.
This kind of air-fuel ratio learning and controlling function can correct an air-fuel ratio according to engine operating condition, and stabilize the air-fuel ratio feedback correction coefficient LMD around the reference value, to thereby improve the controllability of the air-fuel ratio.
An engine operating range is divided into regions based on, for example, basic fuel injection quantities Tp and engine rotation speeds N indicating an engine load, to learn a correction coefficient KBLRC for each of the divided regions.
When the engine operating range for learning the correction coefficients KBLRC is roughly divided, it is impossible to accurately follow the differences of the correction requirements of the respective divided regions. On the other hand, when a very fine division of the engine operating range is made, the ability to learn each of the divided regions is reduced, which deteriorates the convergence of the learning. Also, learned and unlearned regions of the divided regions may be mixed with one another to produce stepwise differences between the respective correction values for the divided regions.
Accordingly, a conventional technique divides the engine operating range into a number of regions by which the learning convergence and the controlling accuracy are improved to some extent. When newly learning the air-fuel ratio of an engine of a just-delivered car or when dealing with a sudden change in a basic air-fuel ratio of an engine due to a fault such as a hole in an inlet system of the engine, it takes time to learn and optimize the correction coefficient KBLRC. Namely, the learning process requires a certain time to reach a target air-fuel ratio, and during this period, the drive-ability and exhaust condition of the engine may be adversely influenced.
The variation of the quantity of intake air of the engine varies is greater when the engine is operating in a low load range than when operating in a high load range, and thus it is preferable to more precisely divide the low load operating range of the engine than the high load operating range thereof, when learning the correction coefficients KBLRC, to ensure an accurate control of the air-fuel ratio of the engine. If a hole is accidentally formed in an inlet system of the engine, air sucked through this hole causes a divergence of the air-fuel ratio, and the extent of the divergence becomes larger as the load on the engine becomes smaller, because a ratio of the air sucked through the hole to the total quantity of intake air becomes larger as the load on the engine becomes smaller. When the engine operating range is divided into small regions, therefore, the learning of the small regions may not progress smoothly, and thus large stepwise differences between the divided regions occur. In the low load region of the engine, in particular, it is difficult to improve the learning convergence and the learning correction accuracy for each engine driving condition.
In consideration of these circumstances, an object of the invention is to provide a method of and an apparatus for learning and controlling the air-fuel ratio of an internal combustion engine, which can properly converge the learning of respective air-fuel ratio correction values of divided regions of an engine operating range, and prevent a stepwise difference between the divided regions due to the air-fuel ratio.